Conventional articles of footwear generally include an upper and a sole structure. The upper provides a covering for the foot and securely positions the foot relative to the sole structure. The sole structure is secured to a lower portion of the upper and is configured so as to be positioned between the foot and the ground when a wearer is standing, walking, or running. The sole structure may include one or more cushioning elements. Those cushioning elements may help to attenuate and dissipate forces on a wearer foot that may result from ground impact during walking or running.
Conventionally, sole structures have been designed based on a particular condition or set of conditions, and/or based on a particular set of preferences and/or characteristics of a targeted shoe wearer. For example, cushioning elements may be sized and located based on expected movements of a shoe wearer associated with a particular type of sport. In many cases, the choice of cushioning elements may be a compromise among numerous possible alternatives. Because of variations among different individuals who might wear a particular shoe, however, some individuals may find a particular compromise to be less than satisfactory. A sole structure that allows adjustment of cushioning characteristics is thus desirable. There is an ongoing need for improved sole structures in which firmness can be modified based on individual wearer preference and/or in response to changing conditions.